1 Field of the Invention
The present invention relates to surface treated bipolar plates for electrochemical cells, particularly polymer electrolyte membrane fuel cells (PEMFC), a process for improvement of the surface properties of such bipolar plates, and fuel cells made with such bipolar plates.
2 Description of the Related Art
A fuel cell converts a fuel such as hydrogen, and an oxidant, typically oxygen or air, in an electrochemical reaction into electricity, reaction products and excess heat. As shown in FIG. 1, a single fuel cell 1 is typically constituted of an electrolyte layer 2 sandwiched between two typically flat porous electrodes 3 and 4, individually referred to as the anode 3 and the cathode 4.
A single polymer electrolyte membrane fuel cell (PEMFC) comprises a thin polymer membrane with high proton conductivity as electrolyte placed between two porous electrodes. The electrode surfaces adjacent to the electrolyte are covered with thin porous layers containing the electrocatalysts typically comprising metals from the platinum group.
Oxidation of hydrogen at the anode 3 catalyst layer generates protons and electrons. The protons are transferred across the electrolyte to the cathode. The electrons travel via an external circuit to the cathode 4. At the cathode 4, oxygen is reduced by consumption of two electrons per atom to form oxide anions which react with the protons that have crossed the electrolyte layer to form water.
A plurality of single cells is usually assembled in a stack to increase the voltage and hence, the power output. Within the stack, adjacent single cells are electrically connected by means of bipolar plates (BPP) 5 and 6 positioned between the surfaces of the electrodes opposite to those contacted with the electrolyte membrane. These BPP must be impermeable for the reactants to prevent their permeation to the opposite electrode, mixing and uncontrolled chemical reaction. With respect to this function, the BPP is often referred to as separator, too. Those BPP or separators can be made of metals, particulate carbon and graphite materials, impregnated graphite or lately also by moulding compounds consisting of graphite and a polymer binder (cf. U.S. Pat. No. 4,214,969). Flow channels or grooves on the surfaces of the BPP provide access for the fuel to the adjacent anode 3 and for the oxidant to the adjacent cathode 4 and removal of the reaction products and the unreacted remnants of fuel and oxidant. These flow channels reduce the useful surface of the BPP, as the electrical contact area is limited to the part of the surface between the channels.
The electrodes 3 and 4 comprise a porous structure referred to as gas diffusion layer (GDL). These GDL have to provide an efficient entry passage for both fuel and oxidant, respectively, to the catalyst layer as well as an exit for the reaction products away from the catalyst layer into the flow channel of the adjacent BPP. To facilitate the mass transfer between the flow channels and the GDL pores, the GDL surface area exposed to the channels should be as large as possible. It is preferred, therefore, that a large portion of the BPP surface is consumed by the flow channels with only a small portion remaining for the electrical contact. Reduction of the electrical contact area is limited, however, by the high contact resistance between BPP and GDL. The contact area between these two must be sufficiently large to avoid local overheating at high current densities which would finally lead to destruction of the assembly. Only a significantly reduced contact resistance between BPP and GDL would allow for a larger channel area and thus better transfer of fuel and oxidant to the electrodes thereby increasing the power output of the fuel cell.
Several suggestions have been made to improve the electronic contact between BPP and GDL, many of them resulting in rather complicated layered structures of the BPP. Those structures (cf. e.g. U.S. Pat. No. 4,956,131) generally comprise an inner layer made of metal or a gas-impermeable conductive carbon material to prevent gas leakage and provide mechanical stability, and outer contact layers made of a porous soft conductive material such as carbon fibres, thermal expansion graphite (cf. EP-A 0 955 686) or carbonaceous dispersed particles (cf. EP-A 1 030 393) to provide good electrical contact to the GDL. It is obvious that the manufacturing of multi-layer BPP is a rather time-consuming and expensive process requiring a more complex technology, compared to the production of a monolithic separator with uniform composition. Therefore it is preferable to create the desired surface properties of the BPP by a rather simple physical or chemical treatment following the process of shaping/moulding or machining.
In the European Patent Application EP-A 0 949 704, a method is described to improve the surface contact between BPP and GDL by immersion of the BPP in acidic solutions. This method, however, involves the utilisation of 30 wt % sulfuric acid and is carried out at 90 xe2x96xa1C. over a long period of time. Such a treatment can attack the polymer binder as well as the graphite material of a BPP and is not suitable for mass production.
Other methods to modify the surface of the BPP as disclosed in EP-A 0 975 040 comprise plasma treatment, corona-discharge treatment and ultraviolet-irradiation treatment each in an atmosphere of hydrophilicising gas. While aimed mainly on improving the hydrophilicity of the BPP surface, most of the examples described there clearly show that the resistivity of the BPP (as measured with the four-probe-method) is negatively affected by the plasma treatment. The resistivity of BPP made by moulding a mixture of phenolic resin and scaly graphite and then subjected to plasma treatment with varying time, output power and hydrophilicising gas was comparable or even significantly higher than that of the untreated BPP made of the same material. Only with increased plasma output power and rather long treatment time a slight decrease of the resistivity (from 15 to 12 mxcexa9xc2x7cm) was achieved. Further shortcomings of this method are the expensive and complex equipment necessary for the plasma or irradiation treatment and the possible destruction of resin particles not only at the surface but also in the bulk of the BPP due to local overheating during plasma treatment.
Consequently a method is required that allows reliable and persistent improvement of the state of the BPP surface employing relatively simple and low cost technique. In the European Patent Application EP-A 0 933 825, a manufacturing method for BPP is disclosed which includes grinding of the press-moulded BPP in order to reduce the contact resistance and to improve the hydrophilicity of the BPP surface. This is not the method of choice since the BPP surface is likely to be contaminated by the grinding agent.
It is an object of the present invention to enhance the conductivity of a BPP especially in the surface region, and thereby minimise the contact resistance between BPP and GDL in a fuel cell assembly. It is a further object to provide an inexpensive method to manufacture BPP with enhanced surface conductivity.
It has now been found that bipolar plates for electrochemical cells comprising a polymer bound conductive material which are devoid of a skin of binder material and which exhibit a through-plane resistivity of not more than 1mxcexa9xc2x7m, preferably less than 0.9 mxcexa9xc2x7m, and especially preferred less than 0.85 mxcexa9xc2x7m, and a surface roughness as measured with a 3 xcexcm front end diameter probe is at least 1.5 xcexcm, and not more than 9 xcexcm have the desired low contact resistance, or high conductivity.
A bipolar plate is said to have a skin of binder material if the partial density of the conductive material in the outer layers which form the flat surface of the plates with a thickness of 5 xcexcm is considerably less than the average partial density of the said conductive material in the overall plate material composition. The partial density in the outer layer is regarded as considerably less if it is less than 70 percent of the overall partial density. Preferably, therefore, a bipolar plate according to this invention exhibits a partial density of conductive material in the outer layer of not less than 80 percent, especially preferred not less than 85 percent of the overall partial density of the said conductive material. Partial density is defined as the ratio of the mass of one component in the mixture and the volume of the mixture. It is further preferred that the deviation from the overall partial density of conductive material in a surface layer with a thickness of 2 xcexcm is so small that it is not less than 70 percent of the overall partial density, or preferably, not less than 80 percent, especially preferred not less than 85 percent of the overall partial density of the said conductive material.
It has also been found that an abrasive treatment which involves exhibiting an untreated plate to a flow of abrasive material which is accelerated into the direction where the plates to be treated are aligned, such as sand-blasting, especially by blasting with an abrasive consisting of inert particles of suitable form and size, provides bipolar plates of the desired through-plane resistivity and surface roughness. This abrasive treatment is applied after complete shaping of the BPP by moulding with subsequent machining, by press-moulding, by injection-moulding or any other state-of-the-art technology.
The abrasive treatment according to this invention results in a reproducible and persistent reduction of the through-plane resistance by at least 30% without mechanical disintegration or destruction of the flow channel structure, without giving rise to gas leakage due to the surface treatment of the plate. Mechanical strength and structural integrity of the BPP are not reduced by the abrasive treatment because by reasonable adjustment of the operation parameters, the thickness reduction is kept in the range of usual manufacturing tolerances. Since the abrasive particles or medium are chosen to be inert towards the constituents of the BPP, the material is not attacked.
Use of the sandblasting technique allows nearly uniform access of the abrasive to all parts of the BPP surface, i.e. protruding lands or fins, and recessed channels, and nearly uniform abrasion can be achieved. Even complicated flow channel structures e.g. containing curvatures are accessible for the abrasive. This is not the case when the abrasion is carried out with a planar tool like in conventional grinding processes.
Another advantage of the method according to the invention is the possibility to perform the surface treatment in a continuous fully-automated process which may be integrated into an automated BPP manufacturing line. This is not possible with other known technologies such as with a plasma treatment because the transfer of the BPP into and out of the plasma chamber introduces discontinuity.